A move by Prime Minister David Cameron to give the British parliament ultimate say over how the European Convention on Human Rights is applied in the UK threatens to unravel Europe’s post-war human rights regime and complicate the UK’s already complicated relations with the European Union.

Chris Grayling, Britain’s justice secretary, said that a Conservative government formed after a general election on 7 May would seek to downgrade rulings by the European Court of Human Rights to merely advisory status.

A Conservative proposal for a British Bill of Rights would empower the House of Commons to overrule any judgment by the court in Strasbourg. The Bill of Rights would replace the current Human Rights Act, which enshrines the Convention in British law. The Conservatives have announced their intention to publish a draft bill before the end of the year, a move that will almost certainly provoke a row with the Liberal Democrats, junior partners in the government.

Any such change would require the consent of all 47 member states of the Council of Europe, the continent’s main human rights watchdog. While many governments would be highly unlikely to give their consent, others might use the UK’s move as an opportunity to reduce the power of European judges in their own countries.

Should the Council of Europe resist a special regime for the UK, the government has threatened to leave the Council of Europe, which would make it the only country in Europe other than Belarus and Kazakhstan to be outside the Council.

In other words, the Conservative proposal would either weaken – possibly fatally – the prevailing human rights regime in Europe, or take the UK out of the Council, which was launched on an initiative by Winston Churchill through the Treaty of London of 1949.

The move could also upset the UK’s relations with the EU. According to officials, it is unclear whether leaving the Council of Europe would have a direct impact on the UK’s membership of the EU. The EU is in the process of acceding to the Convention but this will not happen before the UK’s election next spring.

However, there is a clear presumption that membership of the Council of Europe is a precondition for membership of the European Union. Countries negotiating membership of the Union are required to sign up to the Convention. It would at the very least be extremely awkward to have an existing EU member state withdraw from it.